1. The Field of the Invention
The invention relates to electrical rotating machines wherein the rotor has permanent magnets. More particularly, the invention relates to machines in which the magnets are disposed in recesses in the rotor. The electrical machines in question are commonly called embedded magnet machines. This principle of arrangement for the rotor is widely applied to brushless synchronous machines with flux concentration.
2. The Related Art
The size of an electrical rotating machine depends on its nominal torque. The higher the torque a motor is capable of delivering, the more bulky the electric motor, all other things being equal. There are, however, applications for which it is desirable to obtain both high power levels and a highly compact construction of the motor at the same time. To give just one concrete example, when the intention is to install electric traction motors in the wheels of automotive vehicles, it is desirable to be able to develop power levels of at least 10 kW per motor, and even at least 25 or 30 kW per motor for the majority of the time, for a weight as low as possible in order not to make the unsprung masses too heavy. It is also desirable for the bulk to be very small, going as little as possible beyond the internal volume of the wheel so as not to interfere with the elements of the vehicle in the event of flexing of the suspension and in the event of other types of movement of the wheel with respect to the vehicle body.
These two requirements (high power level, and low weight and bulk) make it very problematic to install electric traction motors in the wheels of private vehicles without a radical improvement in the ratio of weight to power of the electrical machines currently available on the market.
The selection of a high speed at the design stage of an electric motor is one solution which, for a given power, allows the torque and hence the bulk to be reduced. In other words, for a nominal power of a given motor, the greater its nominal speed of rotation, the smaller its bulk will be. However, increasing the speed of rotation of an electrical rotating machine poses numerous problems for the mechanical behavior, and these are particularly difficult if the intention is to keep the weight and bulk of the electrical rotating machine as small as possible.
A specific construction intended to achieve high speeds of rotation has already been proposed in U.S. Pat. No. 6,426,576. The speeds contemplated in this patent application are in the order of 12,000 rpm, and to achieve this a particular arrangement is proposed for the whole assembly comprising a non-magnetic polygonal monobloc shaft and pole pieces which are judiciously disposed around this shaft.
If the speed of rotation is increased further, problems arise with the mechanical behavior of the magnets. The radially outer end thereof is at risk of breaking up. Pieces of magnet are at risk of being spun off, which seems to indicate that internal-rotor machines reach a speed ceiling at the levels of speed of rotation mentioned above.